[PATCH v2 09/13] bash prompt: use bash builtins to get detached HEAD abbrev. object name

["SZEDER Gábor" <szeder@ira.uka.de>]

From: SZEDER Gábor <szeder@ira.uka.de>

When describing a detached HEAD according to the $GIT_PS1_DESCRIBE
environment variable fails, __git_ps1() runs the '$(cut -c1-7
.git/HEAD)' command substitution to put the 7 hexdigits abbreviated
commit object name in the prompt.  This imposes the overhead of
fork()ing a subshell and fork()+exec()ing 'cut'.

Thanks to the previous patch at this point the contents of HEAD has
already been read into a variable, so we can get the 7 hexdigits using
only parameter expansions, sparing the fork()+exec() overhead.

Since zsh doesn't implement substring expansion we can't just use
${head:0:7}, hence the "remove everything except the first 7 chars"
parameter expansion combination.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
---
 contrib/completion/git-prompt.sh | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/contrib/completion/git-prompt.sh b/contrib/completion/git-prompt.sh
index 4e5c8efa..10ec6902 100644
--- a/contrib/completion/git-prompt.sh
+++ b/contrib/completion/git-prompt.sh
@@ -378,9 +378,8 @@ __git_ps1 ()
 				(* | default)
 					git describe --tags --exact-match HEAD ;;
 				esac 2>/dev/null)" ||
-
-				b="$(cut -c1-7 "$g/HEAD" 2>/dev/null)..." ||
-				b="unknown"
+				# detached head abbreviated object name
+				b="${head%${head#???????}}..."
 				b="($b)"
 			fi
 		fi
-- 
1.8.3.1.487.g8f4672d